1. Field
This disclosure relates generally to semiconductors, and more specifically, to a laterally double diffused metal oxide semiconductor (LDMOS) transistor having a reduced surface field structure and method therefor.
2. Related Art
Laterally double diffused metal oxide semiconductor (LDMOS) transistors are used in mixed signal and analog circuits for high power switching. Optimization characteristics such as on resistance (Ron), break-down voltage (BVDSS), and safe operating area (SOA) often conflict. Also, depending on the needs of the particular application, good isolation and high side capabilities may be desirable or necessary. To have high side capability means that a transistor can have all of its terminals coupled to a high potential relative to the substrate, such as when a load is connected to the source of the LDMOS transistor with the drain connected to the supply voltage. Ideally, the performance of a high side capable device does not depend on the voltage between the terminals and the substrate within the rated voltage range. High side capability is desirable because it enables greater flexibility in circuit design, for example, it allows transistors to be stacked. Good isolation is desirable for minimizing substrate injection of charge carriers and enables robustness with respect to negative excursions of the drain-to-substrate voltage. A buried layer can be used to provide isolation from the substrate and to achieve high side capability, as well as for optimizing Ron and BVDSS by providing reduced surface field action (RESURF). However, the buried layer itself can become break-down limiting. Another aspect related to a buried layer is the presence of associated parasitic bipolar transistors (BJT) that may limit the SOA. These problems can be addressed by providing a buried layer that only partially underlies the LDMOS structure; however, isolation and high side capability are sacrificed.
Therefore, what is needed is an LDMOS transistor that solves the above problems.